Electrochemical Deposition of Selenium in Ionic Liquids

ABSTRACT

The invention relates to a process for the electrochemical deposition of grey selenium on a substrate in an ionic liquid, where the electrochemical deposition can be carried out at temperatures of greater than 10000.

The invention relates to a process for the electrochemical deposition of selenium on a substrate in an ionic liquid.

Selenium has a number of properties which make it suitable for many areas of application in photovoltaics and electronics. Selenium is a photosemiconductor which changes (increases) its conductivity by a number of orders of magnitude on exposure to visible light. In addition, selenium is transparent to wavelengths greater than 650 nm.

Selenium is often employed as a constituent in thin-film solar cells in combination with Cu and indium, as described, for example, in DE 44 47 866 or in DE 199 177 58. In the production processes usually mentioned for the production of these systems, either all film components are deposited by evaporation processes or only copper or indium is applied by electroplating. In both cases, the selenium is preferably applied via the gas phase in an evaporator.

Electrochemical deposition of copper indium gallium diselenide is described in US 2004/0206390. The electrochemical deposition there is carried out in a buffered bath comprising potassium biphthalate and amidosulfuric acid. The said components are said to scavenge H⁺ and OH⁻ ions formed by electrolysis. However, the said process is overall unsuitable for the deposition of a single selenium phase.

The electrolytic deposition of selenium is described in U.S. Pat. No. 2,414,438. The deposition is carried out in aqueous alkaline solutions of ammonium selenides, alkali metal selenides or alkaline earth metal selenides.

However, the deposition is made more difficult since amorphous or glass-like red selenium, which has poor electron conductivity, is usually deposited in the room-temperature region. Only at elevated temperature is a grey, metallic phase deposited, but the deposits do not consist uniquely of grey selenium, even at temperatures around 100° C. Higher temperatures, which would promote the deposition of grey selenium, are excluded in open electroplating baths. However, the electrochemical deposition would facilitate continuous operation in the form of vat electroplating without closed evaporators or sputtering devices.

Accordingly, the object of the invention was to find an alternative method for the electrochemical deposition of grey selenium.

The object is achieved by the process according to the invention.

The invention relates to a process for the electrochemical deposition of grey selenium on a substrate in at least one ionic liquid.

The deposition of grey selenium is carried out on a very wide variety of substrates in a very wide variety of applications. The deposition can serve exclusively for the deposition of selenium, but can also be employed in combination with the deposition of further materials, such as, for example, copper or indium.

The ionic liquids which are suitable for the process according to the invention are highly conductive and are generally thermally stable up to 400° C. In particular, use is made of ionic liquids which are electrochemically stable under the deposition conditions described below. They have, for example, an electrochemical window in the cathodic branch which extends from 0 mV to −3500 mV against ferrocene/ferrocinium, preferably from −2000 mV to −3000 mV against ferrocene/ferrocinium. In the anodic branch, suitable ionic liquids have an electrochemical window which extends from 0 mV to +3500 mv against ferrocene/ferrocinium, preferably from +2000 to +3000 mV against ferrocene/ferrocinium. The data relate to the measurement arrangements and conditions described below.

Suitable ionic liquids contain, in particular, at least one tetraalkylammonium or tetraalkylphosphonium cation, where the alkyl groups may each, independently of one another, have 1 to 10 C atoms, or a heterocyclic cation selected from

where

R¹′ to R⁴′ each, independently of one another, denote

hydrogen, —CN, —OR′, —NR′₂, —P(O)R′₂, —P(O)(NR′₂)₂, —C(O)R′,

straight-chain or branched alkyl having 1-20 C atoms,

straight-chain or branched alkenyl having 2-20 C atoms and one or more double bonds,

straight-chain or branched alkynyl having 2-20 C atoms and one or more triple bonds,

saturated, partially or fully unsaturated cycloalkyl having 3-7 C atoms,

which may be substituted by alkyl groups having 1-6 C atoms,

saturated, partially or fully unsaturated heteroaryl,

heteroaryl-C₁-C₆-alkyl or aryl-C₁-C₆-alkyl,

where the substituents R¹′, R²′, R³′ and/or R⁴′ together may also form a ring system,

where one or more substituents R¹′, to R⁴′ may be partially or fully substituted by halogens, in particular —F and/or —Cl, or —OR′, —CN, —C(O)OH, —C(O)NR′₂, —SO₂NR′₂, —C(O)X, —SO₂OH, —SO₂X, —NO₂, but where R¹′ and R⁴′ cannot simultaneously be fully substituted by halogens, and where one or two non-adjacent and non-heteroatom-bonded carbon atoms of the substituents R¹′ to R⁴′ may be replaced by atoms and/or atomic groups selected from —O—, —S—, —S(O)—, —SO₂—, —C(O)—, —N⁺R′₂—, —C(O)NR′—, —SO₂NR′—, —P(O)(NR′₂)NR′—, —PR′₂═N— or —P(O)R′—, where R′═H, non-, partially or perfluorinated C₁— to C₆-alkyl, C₃— to C₇-cycloalkyl, unsubstituted or substituted phenyl, and X=halogen.

The substituent R²′ or R³′ is in each case, independently of one another, in particular hydrogen, methyl, ethyl, isopropyl, propyl, butyl, sec-butyl, tert-butyl, cyclohexyl, phenyl or benzyl. R²′ is particularly preferably hydrogen, methyl, ethyl, isopropyl, propyl, butyl or secbutyl. R²′ and R³′ are very particularly preferably hydrogen.

The C₁-C₁₂-alkyl group is, for example, methyl, ethyl, isopropyl, propyl, butyl, sec-butyl or tert-butyl, furthermore also pentyl, 1-, 2- or 3-methylbutyl, 1,1-, 1,2- or 2,2-dimethylpropyl, 1-ethylpropyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl or dodecyl. Optionally difluoromethyl, trifluoromethyl, pentafluoroethyl, heptafluoropropyl or nonafluorobutyl.

A straight-chain or branched alkenyl having 2 to 20 C atoms, in which, in addition, a plurality of double bonds may be present, is, for example, allyl, 2- or 3-butenyl, isobutenyl, sec-butenyl, furthermore 4-pentenyl, isopentenyl, hexenyl, heptenyl, octenyl, —C₉H₁₇, —C₁₀H₁₉ to —C₂₀H₃₉; preferably allyl, 2- or 3-butenyl, isobutenyl, sec-butenyl, preference is furthermore given to 4-pentenyl, isopentenyl or hexenyl.

A straight-chain or branched alkynyl having 2 to 20 C atoms, in which, in addition, a plurality of triple bonds may be present, is, for example, ethynyl, 1- or 2-propynyl, 2- or 3-butynyl, furthermore 4-pentynyl, 3-pentynyl, hexynyl, heptynyl, octynyl, —C₉H₁₅, —C₁₀H₁₇ to —C₂₀H₃₇, preferably ethynyl, 1- or 2-propynyl, 2- or 3-butynyl, 4-pentynyl, 3-pentynyl or hexynyl.

Aryl-C₁-C₆-alkyl denotes, for example, benzyl, phenylethyl, phenylpropyl, phenylbutyl, phenylpentyl or phenylhexyl, where both the phenyl ring and also the alkylene chain may, as described above, be partially or fully substituted by halogens, in particular —F and/or —Cl, or partially by —OR′, —CN, —C(O)OH, —C(O)NR′₂, —SO₂NR′₂, —C(O)X, —SO₂OH, —SO₂X, —NO₂.

Unsubstituted saturated or partially or fully unsaturated cycloalkyl groups having 3-7 C atoms are therefore cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclopentenyl, cyclopenta-1,3-dienyl, cyclohexenyl, cyclohexa-1,3-dienyl, cyclohexa-1,4-dienyl, phenyl, cycloheptenyl, cyclohepta-1,3-dienyl, cyclohepta-1,4-dienyl or cyclohepta-1,5-dienyl, each of which may be substituted by C₁— to C₆-alkyl groups, where the cycloalkyl group or the cycloalkyl group which is substituted by C₁— to C₆-alkyl groups may in turn also be substituted by halogen atoms, such as F, Cl, Br or I, in particular F or Cl, or by —OR′, —CN, —C(O)OH, —C(O)NR′₂, —SO₂NR′₂, —C(O)X, —SO₂OH, —SO₂X, —NO₂.

In the substituents R¹′ to R⁴′, one or two carbon atoms which are not adjacent and are not bonded in the α-position to the heteroatom may also be replaced by atoms and/or atomic groups selected from the group —O—, —S—, —S(O)—, —SO₂—, —N⁺R′₂—, —C(O)NR′—, —SO₂NR′—, —P(O)(NR′₂)NR′— or —P(O)R′—, where R′=non-, partially or perfluorinated C₁— to C₆-alkyl, C₃— to C₇-cycloalkyl, unsubstituted or substituted phenyl.

Without restricting generality, examples of substituents R¹′ to R⁴′ modified in this way are: —OCH₃, —OCH(CH₃)₂, —CH₂OCH₃, —CH₂—CH₂—O—CH₃, —C₂H₄OCH(CH₃)₂, —C₂H₄SC₂H₅, —C₂H₄SCH(CH₃)₂, —S(O)CH₃, —SO₂CH₃, —SO₂C₆H₅, —SO₂C₃H₇, —SO₂CH(CH₃)₂, —SO₂CH₂CF₃, —CH₂SO₂CH₃, —O—C₄H₈—O—C₄H₉, —CF₃, —C₂F₅, —C₃F₇, —C₄F₉, —C(CF₃)₃, —CF₂SO₂CF₃, —C₂F₄N(C₂F₅)C₂F₅, —CHF₂, —CH₂CF₃, —C₂F₂H₃, —C₃FH₆, —CH₂C₃F₇, —C(CFH₂)₃, —CH₂C(O)OH, —CH₂C₆H₅ or P(O)(C₂H₅)₂.

In R′, C₃— to C₇-cycloalkyl is, for example, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl or cycloheptyl.

In R′, substituted phenyl denotes phenyl which is substituted by C₁— to C₆-alkyl, C₁— to C₆-alkenyl, NO₂, F, Cl, Br, I, C₁-C₆-alkoxy, SCF₃, SO₂CF₃, COOH, SO₂X′, SO₂NR″₂ or SO₃H, where X′ denotes F, Cl or Br and R″ denotes a non-, partially or perfluorinated C₁— to C₆-alkyl or C₃— to C₇-cycloalkyl as defined for R′, for example, o-, m- or p-methylphenyl, o-, m- or p-ethylphenyl, o-, m- or p-propylphenyl, o-, m- or p-isopropylphenyl, o-, m- or p-tert-butylphenyl, o-, m- or p-nitrophenyl, o-, m- or p-methoxyphenyl, o-, m- or p-ethoxyphenyl, o-, m-, p-(trifluoromethyl)phenyl, o-, m-, p-(trifluoromethoxy)phenyl, o-, m-, p-(trifluoromethylsulfonyl)phenyl, o-, m- or p-fluorophenyl, o-, m- or p-chlorophenyl, o-, m- or p-bromophenyl, o-, m- or p-iodophenyl, furthermore preferably 2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-dimethylphenyl, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-dihydroxyphenyl, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-difluorophenyl, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-dichlorophenyl, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-dibromophenyl, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-dimethoxyphenyl, 5-fluoro-2-methylphenyl, 3,4,5-trimethoxyphenyl or 2,4,5-trimethylphenyl.

In R¹′ to R⁴′, heteroaryl is taken to mean a saturated or unsaturated mono- or bicyclic heterocyclic radical having 5 to 13 ring members, where 1, 2 or 3 N and/or 1 or 2 S or O atoms may be present and the heterocyclic radical may be mono- or polysubstituted by C₁— to C₆-alkyl, C₁— to C₆-alkenyl, NO₂, F, Cl, Br, I, C₁-C₆-alkoxy, SCF₃, SO₂CF₃, COOH, SO₂X′, SO₂NR″₂ or SO₃H, where X′ and R″ have a meaning indicated above. The heterocyclic radical is preferably substituted or unsubstituted 2- or 3-furyl, 2- or 3-thienyl, 1-, 2- or 3-pyrrolyl, 1-, 2-, 4- or 5-imidazolyl, 3-, 4- or 5-pyrazolyl, 2-, 4- or 5-oxazolyl, 3-, 4- or 5-isoxazolyl, 2-, 4- or 5-thiazolyl, 3-, 4- or 5-isothiazolyl, 2-, 3- or 4-pyridyl, 2-, 4-, 5- or 6-pyrimidinyl, furthermore preferably 1,2,3-triazol-1-, -4- or -5-yl, 1,2,4-triazol-1-, -4- or -5-yl, 1- or 5-tetrazolyl, 1,2,3-oxadiazol-4- or -5-yl 1,2,4-oxadiazol-3- or -5-yl, 1,3,4-thiadiazol-2- or -5-yl, 1,2,4-thiadiazol-3- or -5-yl, 1,2,3-thiadiazol-4- or -5-yl, 2-, 3-, 4-, 5- or 6-2h-thiopyranyl, 2-, 3- or 4-4h-thiopyranyl, 3- or 4-pyridazinyl, pyrazinyl, 2-, 3-, 4-, 5-, 6- or 7-benzofuryl, 2-, 3-, 4-, 5-, 6- or 7-benzothienyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-1H-indolyl, 1-, 2-, 4- or 5-benzimidazolyl, 1-, 3-, 4-, 5-, 6- or 7-benzopyrazolyl, 2-, 4-, 5-, 6- or 7-benzoxazolyl, 3-, 4-, 5-, 6- or 7-benzisoxazolyl, 2-, 4-, 5-, 6- or 7-benzothiazolyl, 2-, 4-, 5-, 6- or 7-benzisothiazolyl, 4-, 5-, 6- or 7-benz-2,1,3-oxadiazolyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- or 8-quinolinyl, 1-, 3-, 4-, 5-, 6-, 7- or 8-isoquinolinyl, 1-, 2-, 3-, 4- or 9-carbazolyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-acridinyl, 3-, 4-, 5-, 6-, 7- or 8-cinnolinyl, 2-, 4-, 5-, 6-, 7- or 8-quinazolinyl or 1-, 2- or 3-pyrrolidinyl.

Heteroaryl-C₁-C₆-alkyl is now, analogously to aryl-C₁-C₆-alkyl, taken to mean for example pyridinylmethyl, pyridinylethyl, pyridinylpropyl, pyridinylbutyl, pyridinylpentyl, pyridinylhexyl, where furthermore the heterocyclic radicals described above may be linked to the alkylene chain in this way.

R¹′ to R⁴′ are, in particular, alkyl groups having 1 to 10 C atoms or hydroxyalkyl groups having 1 to 10 C atoms.

Particularly suitable ionic liquids contain a tetraalkylammonium, tetraalkylphosphonium, 1,1-dialkylpyrrolidinium, 1-hydroxyalkyl-1-alkylpyrrolidinium, 1-hydroxyalkyl-3-alkylimidazolium or 1,3-bis(hydroxyalkyl)imidazolium cation, where the alkyl groups or the alkylene chain of the hydroxyalkyl group may each, independently of one another, have 1 to 10 C atoms.

An alkyl group having 1 to 10 C atoms is taken to mean, for example, methyl, ethyl, isopropyl, propyl, butyl, sec-butyl or tert-butyl, furthermore also pentyl, 1-, 2- or 3-methylbutyl, 1,1-, 1,2- or 2,2-dimethylpropyl, 1-ethylpropyl, hexyl, heptyl, octyl, nonyl or decyl. The alkyl groups may also be partially or fully substituted by fluorine. Fluorinated alkyl groups are, for example, difluoromethyl, trifluoromethyl, pentafluoroethyl, pentafluoropropyl, heptafluoropropyl, heptafluorobutyl or nonafluorobutyl.

A hydroxyalkyl group having 1 to 10 C atoms is taken to mean, for example, 1-hydroxymethyl, 2-hydroxyethyl, 3-hydroxyropyl, 4-hydroxybutyl, furthermore also 5-hydroxypentyl, 6-hydroxyhexyl, 7-hydroxyheptyl, 8-hydroxyoctyl, 9-hydroxynonyl or 10-hydroxydecyl. The alkylene chain of the hydroxyl group may also be partially or fully substituted by fluorine. Fluorinated hydroxyalkyl groups can be described, for example, by the sub-formulae —(CHF)_(n)—OH or —(CF₂)_(n)—OH, where n can denote 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

Suitable anions which, in combination with the cations according to the invention, satisfy the above-mentioned condition with respect to stability can be selected from the group PF₆, BF₄, alkylsulfate, perfluoroalkylsulfonate, perfluoroacetate, bis(fluorosulfonyl)imide, bis(perfluoroalkylsulfonyl)imide, tris(perfluoroalkyl)trifluorophosphate, bis(perfluoroalkyl)tetrafluorophosphate, tris(perfluoroalkylsulfonyl)methide or perfluoroalkylborate.

The term perfluoroalkyl group means that all H atoms of the corresponding alkyl group have been replaced by F atoms. The alkyl or perfluoroalkyl groups in the anions indicated preferably each, independently of one another, have 1 to 10 C atoms, particularly preferably 1, 2, 3 or 4 C atoms.

Anions which are suitable in accordance with the invention can be selected, from the group trifluoromethylsulfonate, pentafluoroethylsulfonate, heptafluoropropylsulfonate, nonafluorobutylsulfonate, bis(fluorosulfonyl)imide, perfluoroacetate, bis(trifluoromethylsulfonyl)imide, bis(pentafluoroethylsulfonyl)imide, bis(heptafluoropropylsulfonyl)imide, bis(nonafluorobutylsulfonyl)imide, tris(trifluoromethylsulfonyl)methide, tris(pentafluoroethylsulfonyl)methide, tris(heptafluoropropylsulfonyl)methide, tris(nonafluorobutylsulfonyl)methide, tris(pentafluoroethyl)trifluorophosphate, tris(heptafluoropropyl)trifluorophosphate, tris(nonafluorobutyl)trifluorophosphate, bis(pentafluoroethyl)tetrafluorophosphate, tetrakis(trifluoromethyl)borate, tetrakis(pentafluoroethyl)borate, trifluoromethyltrifluoroborate, pentafluoroethyltrifluoroborate, bis(trifluoromethyl)difluoroborate, bis(pentafluoroethyl)difluoroborate, tris(trifluoromethyl)fluoroborate, tris(pentafluoroethyl)fluoroborate or bis(pentafluoroethyl)trifluoromethylfluoroborate.

As soon as a plurality of perfluoroalkyl groups occur in the anions, these can denote, independently of one another, different perfluoroalkyl groups. The above-mentioned definition therefore also covers, for example, mixed anions, such as trifluoromethylsulfonyl-pentafluoroethylsulfonylimide, bis(trifluoromethyl)sulfonyl-pentafluoroethylsulfonylmethide.

The anions are particularly preferably selected from the group trifluoromethanesulfonate, bis(trifluoromethylsulfonyl)imide or tris(pentafluoroethyl)trifluorophosphate.

Suitable cations are, in particular, optionally linear or branched, tetramethylammonium, tetraethylammonium, tetrapropylammonium, tetrabutylammonium, tetrapentylammonium, tetrahexylammonium, tetraheptylammonium, tetraoctylammonium, tetranonylammonium, tetradecylammonium, trimethylalkylammonium, trimethyl(ethyl)ammonium, triethyl(methyl)ammonium, trihexylammonium, methyl(trioctyl)ammonium, tetramethylphosphonium, tetraethylphosphonium, tetrapropylphosphonium, tetrabutylphosphonium, tetrapentylphosphonium, tetrahexylphosphonium, tetraheptylphosphonium, tetraoctylphosphonium, tetranonylphosphonium, tetradecylphosphonium, trihexyltetradecylphosphonium, triisobutyl(methyl)phosphonium, tributyl(ethyl)phosphonium, tributyl(methyl)phosphonium, 1,1-dimethylpyrrolidinium, 1-methyl-1-ethylpyrrolidinium, 1-methyl-1-propylpyrrolidinium, 1-methyl-1-butylpyrrolidinium, 1-methyl-1-pentylpyrrolidinium, 1-methyl-1-hexylpyrrolidinium, 1-methyl-1-heptylpyrrolidinium, 1-methyl-1-octylpyrrolidinium, 1-methyl-1-nonylpyrrolidinium, 1-methyl-1-decylpyrrolidinium, 1,1-diethylpyrrolidinium, 1-ethyl-1-propylpyrrolidinium, 1-ethyl-1-butylpyrrolidinium, 1-ethyl-1-pentylpyrrolidinium, 1-ethyl-1-hexylpyrrolidinium, 1-ethyl-1-heptylpyrrolidinium, 1-ethyl-1-octylpyrrolidinium, 1-ethyl-1-nonylpyrrolidinium, 1-ethyl-1-decylpyrrolidinium, 1,1-dipropylpyrrolidinium, 1-propyl-1-butylpyrrolidinium, 1-propyl-1-pentylpyrrolidinium, 1-propyl-1-hexylpyrrolidinium, 1-propyl-1-heptylpyrrolidinium, 1-propyl-1-octylpyrrolidinium, 1-propyl-1-nonylpyrrolidinium, 1-propyl-1-decylpyrrolidinium, 1,1-dibutylpyrrolidinium, 1-butyl-1-pentylpyrrolidinium, 1-butyl-1-hexylpyrrolidinium, 1-butyl-1-heptylpyrrolidinium, 1-butyl-1-octylpyrrolidinium, 1-butyl-1-nonylpyrrolidinium, 1-butyl-1-decylpyrrolidinium, 1,1-dipentylpyrrolidinium, 1-pentyl-1-hexylpyrrolidinium, 1-pentyl-1-heptylpyrrolidinium, 1-pentyl-1-octylpyrrolidinium, 1-pentyl-1-nonylpyrrolidinium, 1-pentyl-1-decylpyrrolidinium, 1,1-dihexylpyrrolidinium, 1-hexyl-1-heptylpyrrolidinium, 1-hexyl-1-octylpyrrolidinium, 1-hexyl-1-nonylpyrrolidinium, 1-hexyl-1-decylpyrrolidinium, 1,1-dihexylpyrrolidinium, 1-hexyl-1-heptylpyrrolidinium, 1-hexyl-1-octylpyrrolidinium, 1-hexyl-1-nonylpyrrolidinium, 1-hexyl-1-decylpyrrolidinium, 1,1-diheptylpyrrolidinium, 1-heptyl-1-octylpyrrolidinium, 1-heptyl-1-nonylpyrrolidinium, 1-heptyl-1-decylpyrrolidinium, 1,1-dioctylpyrrolidinium, 1-octyl-1-nonylpyrrolidinium, 1-octyl-1-decylpyrrolidinium, 1-1-dinonylpyrrolidinium, 1-nonyl-1-decylpyrrolidinium or 1,1-didecylpyrrolidinium, 1-hydroxymethyl-1-methylpyrrolidinium, 1-hydroxymethyl-1-ethylpyrrolidinium, 1-hydroxymethyl-1-propylpyrrolidinium, 1-hydroxymethyl-1-butylpyrrolidinium, 1-(2-hydroxyethyl)-1-methylpyrrolidinium, 1-(2-hydroxyethyl)-1-ethylpyrrolidinium, 1-(2-hydroxyethyl)-1-propylpyrrolidinium, 1-(2-hydroxyethyl)-1-butylpyrrolidinium, 1-(3-hydroxypropyl)-1-methylpyrrolidinium, 1-(3-hydroxypropyl)-1-ethylpyrrolidinium, 1-(3-hydroxypropyl)-1-propylpyrrolidinium, 1-(3-hydroxypropyl)-1-butylpyrrolidinium, 1-(4-hydroxybutyl)-1-methylpyrrolidinium, 1-(4-hydroxybutyl)-1-ethylpyrrolidiniurn, 1-(4-hydroxybutyl)-1-propylpyrrolidinium or 1-(4-hydroxybutyl)-1-butylpyrrolidinium, 1-(1-hydroxymethyl)-3-methylimidazolium, 1-(1-hydroxymethyl)-3-ethylimidazolium, 1-(1-hydroxymethyl)-3-propylimidazolium, 1-(1-hydroxymethyl)-3-butylimidazolium, 1-(2-hydroxyethyl)-3-methylimidazolium, 1-(2-hydroxyethyl)-3-ethylimidazolium, 1-(2-hydroxyethyl)-3-propylimidazolium, 1-(2-hydroxyethyl)-3-butylimidazolium, 1-(3-hydroxypropyl)-3-methylimidazolium, 1-(3-hydroxypropyl)-3-ethylimidazolium, 1-(3-hydroxypropyl)-3-propylimidazolium, 1-(3-hydroxypropyl)-3-butylimidazolium, 1-(4-hydroxybutyl)-3-methylimidazolium, 1-(4-hydroxybutyl)-3-ethylimidazolium, 1-(4-hydroxybutyl)-3-propylimidazolium, 1-(4-hydroxybutyl)-3-butylimidazolium, 1,3-bis(1-hydroxymethyl)imidazolium, 1,3-bis(2-hydroxyethyl)imidazolium, 1,3-bis(3-hydroxypropyl)imidazolium, 1,3-bis(4-hydroxybutyl)imidazolium, 1-(2-hydroxyethyl)-3-(1-hydroxymethyl)imidazolium, 1-(2-hydroxyethyl)-3-(3-hydroxypropyl)imidazolium, 1-(2-hydroxyethyl)-3-(4-hydroxybutyl)imidazolium, 1-(3-hydroxypropyl)-3-(1-hydroxymethyl)imidazolium, 1-(3-hydroxypropyl)-3-(2-hydroxyethyl)imidazolium, 1-(3-hydroxypropyl)-3-(4-hydroxybutyl)imidazolium, 1-(4-hydroxybutyl)-3-(1-hydroxymethyl)imidazolium, 1-(4-hydroxybutyl)-3-(2-hydroxyethyl)imidazolium or 1-(4-hydroxybutyl)-3-(3-hydroxypropyl)imidazolium.

Particularly suitable cations are tetramethylammonium, trimethylalkylammonium, where the alkyl group can have 1 to 10 C atoms, in particular trihexyltetradecylphosphonium, triisobutyl(methyl)phosphonium, tributyl(ethyl)phosphonium, tributyl(methyl)phosphonium, in addition preferably 1-butyl-1-methylpyrrolidinium, 1-butyl-1-ethylpyrrolidinium, 1-hexyl-1-methylpyrrolidinium, 1-methyl-1-octylpyrrolidinium or 1-(2-hydroxyethyl)-3-methylimidazolium, very particularly suitable cations are 1-butyl-1-methylpyrrolidinium, 1-hexyl-1-methylpyrrolidinium, 1-methyl-1-octylpyrrolidinium or 1-(2-hydroxyethyl)-3-methyl-imidazolium.

Particularly suitable ionic liquids for use in the process according to the invention are

1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate,

1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide,

1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate,

1-hexyl-1-methylpyrrolidinium trifluoromethanesulfonate,

1-hexyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide,

1-hexyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate,

1-methyl-1-octylpyrrolidinium trifluoromethanesulfonate,

1-methyl-1-octylpyrrolidinium bis(trifluoromethylsulfonyl)imide,

1-methyl-1-octylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate,

1-(2-hydroxyethyl)-3-methylimidazolium trifluoromethanesulfonate,

1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or

1-(2-hydroxyethyl)-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate.

According to the process according to the invention, selenium ions are dissolved in a suitable ionic liquid, as described above.

This can be carried out by dissolution of a selenium salt in the ionic liquid.

Suitable selenium salts are, for example, selenium tetrahalides, for example selenium tetrachloride or selenium tetrabromide, but also selenium dioxide. In principle, any selenium salt which facilitates selenium deposition under the said conditions is suitable.

The ion concentration in the ionic liquid for the deposition of metal is preferably 10⁻⁵ to 10 mol/l. An ion concentration of 10⁻³ to 10⁻¹ mol/l is preferably used.

The deposition according to the invention is carried out in a protective-gas atmosphere, for example under argon, where the oxygen and water content and in particular the water content should be below 1 ppm. The deposition is carried out in a 3-electrode cell, as known to the person skilled in the art (for example from A. J. Bard, L. R. Faulkner, Electrochemical Methods, Wiley). During deposition on a suitable substrate, platinum or selenium electrodes are preferably used as counter- and reference electrodes. In general, all metals or carbon can be employed as electrode materials so long as the products formed at the counterelectrode do not interfere with the processes at the working electrode, i.e. so long as the electrode materials are not deposited together with selenium under the experimental conditions. The choice of suitable materials here is made within the expert knowledge of the person skilled in the art.

The process according to the invention is preferably carried out potentiostatically, at electrode potentials between 0 and −2000 mV and at temperatures between 10° C. and 230° C., preferably between 100° C. to 150° C.

However, the process according to the invention can also be carried out by means of pulsed techniques, as are known to the person skilled in the art, for example as described in J.-C. Puippe, F. Leaman, Pulse-Plating: Elektrolytische Metallabscheidung mit Pulsstrom [Pulse Plating: Electrolytic Deposition of Metal by Means of a Pulsed Current], Eugen G. Leuze Verlag, 1990.

The process according to the invention enables grey selenium to be deposited in any desired layer thicknesses, for example between 200 μm and 300 μm, in continuous layers made up of micro- or nanocrystals. The desired layer thickness is controlled via the electrode potential and the charge that has flowed as well as the electrochemical parameters.

This correlation is described in a generally valid manner via the Faraday law:

${d = \frac{I*t*M}{F*A*\rho}},$

where F=Faraday constant, A=area, ρ=density of the metal, I=current, t=time and M=molar mass of the metal.

Ultimately, the layer thickness can be set via the current and time.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cyclic voltammogram of an approximately 0.1 molar solution of SeCl₄ in 1-butyl-1-methylpyrrolid inium bis(trifluoromethylsulfonyl)imide (BMP Tf₂N) at room temperature on Au(111). A first reduction peak occurs at −1 V vs. Pt quasi-reference and a second occurs at −1.75 V. An oxidation process is evident at +0.5 volt, where the ratio of the anodic to the cathodic currents that have flowed is significantly less than 1.

At 120° C. (FIG. 2), the shape of the cyclic voltammogram changes significantly. Instead of 2 clear reduction processes, only one cathodic current increasing to a greater or lesser extent with a pronounced oxidation process at about 1 volt vs. Pt quasi-reference is observed.

FIG. 3 shows the morphology of selenium deposits which have been deposited from SeCl₄ in BMP Tf2N. at 150° C.

FIG. 4 shows an X-ray diffraction pattern (XRD=X-ray diffraction, cobalt K alpha as X-ray) of selenium which has been deposited from SeCl₄/BMP Tf₂N [0.1 mol/l of SeCl₄ in BMP Tf₂N] at 150° C. on platinum.

A wide variety of substrates which can be employed as cathode are possible for the electrochemical deposition of selenium according to the invention. The geometry of the substrates is freely selectable and is not subject to any restriction.

Suitable substrates can be selected, for example, from all categories, for example non-metals, semi-metals, metals, metal alloys, conductive or metallized ceramics or conductive or metallized plastics are possible.

A preferred non-metal is, for example, graphite.

A preferred semi-metal is, for example, silicon.

Preferred metals are, for example, gold, platinum, copper, iron, cobalt, nickel or molybdenum.

Preferred metal alloys are, for example, a very wide variety of steels or nickel alloys.

Suitable substrates may also already consist, for example, of a plurality of layers to which a further layer of selenium is applied as interlayer or final layer by the process according to the invention. The list of substrates should therefore in no way be regarded as limiting. The person skilled in the art in the respective area of application will be able to select the suitable substrate without further information.

After deposition of selenium, the ionic liquid can be washed out using organic solvents. Suitable organic solvents are, for example, toluene, benzene, methylene chloride, acetonitrile, acetone, methanol, ethanol or isopropanol. On use of selenium tetrachloride, this can also be removed, for example, from the ionic liquid by heating under reduced pressure by distillation.

The present invention likewise relates to substrates which are coated using the process according to the invention. The present invention furthermore relates to the use of ionic liquids for the deposition, preferably for the electrochemical deposition, of grey selenium on a substrate.

The detailed conditions for the deposition and the suitable ionic liquids and the aftertreatment of the coated substrate are revealed in the comments described above.

Even without further comments, it is assumed that a person skilled in the art will be able to utilize the above description in the broadest scope. The preferred embodiments and examples should therefore merely be regarded as descriptive disclosure which is absolutely not limiting in any way.

EXAMPLES

The electrochemical measurements were carried out using a Princeton Applied Research (EG & G) PAR 2263 potentiostat/galvanostat. In general, any potentiostat and any constant-current source as well as batteries, in each case with or without pulse generator, are suitable.

Example 1 Deposition of Selenium from SeCl₄ on Gold

A solution of SeCl₄ in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide is prepared and transferred into the 3-electrode measurement cell at room temperature under a protective-gas atmosphere. A typical 3-electrode measurement cell was used, as described, for example, in A. J. Bard and L. R. Faulkner, Electrochemical Methods, Wiley. The 3-electrode measurement cell has a gold electrode as working electrode (cathode), and platinum wires serve as quasi-reference and counter-electrode.

The electrode potential is set to −1500 mV vs. platinum quasi-reference.

The deposition of selenium begins at −1000 mV.

Example 2 Deposition of Selenium from SeBr₄ on Platinum

A 0.25 molar solution of SeBr₄ in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide is prepared analogously to Example 1 and transferred into the 3-electrode measurement cell at room temperature under a protective-gas atmosphere. A typical 3-electrode measurement cell was used, as described, for example, in A. J. Bard and L. R. Faulkner, Electrochemical Methods, Wiley.

The 3-electrode measurement cell has a platinum electrode as working electrode (cathode), and platinum wires serve as quasi-reference and counterelectrode.

The electrode potential is set to −1500 mV vs. platinum quasi-reference.

The deposition of selenium begins at −1000 mV.

Example 3 Deposition of Selenium from SeBr₄ on Indium

A 0.25 molar solution of SeBr₄ in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide is prepared analogously to Example 1 and transferred into the 3-electrode measurement cell at room temperature under a protective-gas atmosphere. A typical 3-electrode measurement cell was used, as described, for example, in A. J. Bard and L. R. Faulkner, Electrochemical Methods, Wiley.

The 3-electrode measurement cell has an indium electrode as working electrode (cathode), and platinum wires serve as quasi-reference and counterelectrode.

The electrode potential is set to −1500 mV vs. platinum quasi-reference.

The deposition of selenium begins at −1000 mV. 

1. Process for the electrochemical deposition of grey selenium on a substrate in at least one ionic liquid.
 2. Process according to claim 1, characterized in that the ionic liquid has at least one tetraalkylammonium or tetraalkylphosphonium cation, where the alkyl groups may each, independently of one another, have 1 to 10 C atoms, or a heterocyclic cation selected from

where R¹′ to R⁴′ each, independently of one another, denote hydrogen, —CN, —OR′, —NR′₂, —P(O)R′₂, —P(O)(NR′₂)₂, —C(O)R′, straight-chain or branched alkyl having 1-20 C atoms, straight-chain or branched alkenyl having 2-20 C atoms and one or more double bonds, straight-chain or branched alkynyl having 2-20 C atoms and one or more triple bonds, saturated, partially or fully unsaturated cycloalkyl having 3-7 C atoms, which may be substituted by alkyl groups having 1-6 C atoms, saturated, partially or fully unsaturated heteroaryl, heteroaryl-C₁-C₆-alkyl or aryl-C₁-C₆-alkyl, where the substituents R¹′, R²′, R³′ and/or R⁴′ together may also form a ring system, where one or more substituents R¹′ to R⁴′ may be partially or fully substituted by halogens, in particular —F and/or —Cl, or —OR′, —CN, —C(O)OH, —C(O)NR′₂, —SO₂NR′₂, —C(O)X, —SO₂OH, —SO₂X, —NO₂, but where R¹′ and R⁴′ cannot simultaneously be fully substituted by halogens, and where one or two non-adjacent and non-heteroatom-bonded carbon atoms of the substituents R¹′ to R⁴′ may be replaced by atoms and/or atomic groups selected from —O—, —S—, —S(O)—, —SO₂—, —C(O)—, —N⁺R′₂—, —C(O)NR′—, —SO₂NR′—, —P(O)(NR′₂)NR′—, —PR′₂═N— or —P(O)R′—, where R′═H, non-, partially or perfluorinated C₁— to C₆-alkyl, C₃— to C₇-cycloalkyl, unsubstituted or substituted phenyl, and X=halogen.
 3. Process according to claim 1, characterized in that the ionic liquid contains at least one tetraalkylammonium, tetraalkylphosphonium-, 1,1-dialkylpyrrolidinium, 1-hydroxyalkyl-1-alkylpyrrolidinium, 1-hydroxyalkyl-3-alkylimidazolium or 1,3-bis(hydroxyalkyl)imidazolium cation, where the alkyl groups or the alkylene chain of the hydroxyalkyl group may each, independently of one another, have 1 to 10 C atoms.
 4. Process according to claim 1, characterized in that the ionic liquid has an electrochemical window from 0 mV to −3500 mV against ferrocene/ferrocinium in the cathodic branch and from 0 mV to +3500 mV against ferrocene/ferrocinium in the anodic branch.
 5. Process according to claim 1, characterized in that the anion of the ionic liquid is selected from the group PF₆, BF₄, alkylsulfate, perfluoroalkylsulfonate, perfluoroacetate, bis(fluorosulfonyl)imide, bis(perfluoroalkylsulfonyl)imide, tris(perfluoroalkyl)trifluorophosphate, bis(perfluoroalkyl)tetrafluorophosphate, tris(perfluoroalkylsulfonyl)methide or perfluoroalkylborate.
 6. Process according to claim 1, characterized in that the cation is selected from the group linear or branched tetramethylammonium, tetraethylammonium, tetrapropylammonium, tetrabutylammonium, tetrapentylammonium, tetrahexylammonium, tetraheptylammonium, tetraoctylammonium, tetranonylammonium, tetradecylammonium, trimethylalkylammonium, trimethyl(ethyl)ammonium, triethyl(methyl)ammonium, trihexylammonium, methyl(trioctyl)ammonium, tetramethylphosphonium, tetraethylphosphonium, tetrapropylphosphonium, tetrabutylphosphonium, tetrapentylphosphonium, tetrahexylphosphonium, tetraheptylphosphonium, tetraoctylphosphonium, tetranonylphosphonium, tetradecylphosphonium, trihexyltetradecylphosphonium, triisobutyl(methyl)phosphonium, tributyl(ethyl)phosphonium, tributyl(methyl)phosphonium, 1,1-dimethylpyrrolidinium, 1-methyl-1-ethylpyrrolidinium, 1-methyl-1-propylpyrrolidinium, 1-methyl-1-butylpyrrolidinium, 1-methyl-1-pentylpyrrolidinium, 1-methyl-1-hexylpyrrolidinium, 1-methyl-1-heptylpyrrolidinium, 1-methyl-1-octylpyrrolidinium, 1-methyl-1-nonylpyrrolidinium, 1-methyl-1-decylpyrrolidinium, 1,1-diethylpyrrolidinium, 1-ethyl-1-propylpyrrolidinium, 1-ethyl-1-butylpyrrolidinium, 1-ethyl-1-pentylpyrrolidinium, 1-ethyl-1-hexylpyrrolidinium, 1-ethyl-1-heptylpyrrolidinium, 1-ethyl-1-octylpyrrolidinium, 1-ethyl-1-nonylpyrrolidinium, 1-ethyl-1-decylpyrrolidinium, 1,1-dipropylpyrrolidinium, 1-propyl-1-butylpyrrolidinium, 1-propyl-1-pentylpyrrolidinium, 1-propyl-1-hexylpyrrolidinium, 1-propyl-1-heptylpyrrolidinium, 1-propyl-1-octylpyrrolidinium, 1-propyl-1-nonylpyrrolidinium, 1-propyl-1-decylpyrrolidinium, 1,1-dibutylpyrrolidinium, 1-butyl-1-pentylpyrrolidinium, 1-butyl-1-hexylpyrrolidinium, 1-butyl-1-heptylpyrrolidinium, 1-butyl-1-octylpyrrolidinium, 1-butyl-1-nonylpyrrolidinium, 1-butyl-1-decylpyrrolidinium, 1,1-dipentylpyrrolidinium, 1-pentyl-1-hexylpyrrolidinium, 1-pentyl-1-heptylpyrrolidinium, 1-pentyl-1-octylpyrrolidinium, 1-pentyl-1-nonylpyrrolidinium, 1-pentyl-1-decylpyrrolidinium, 1,1-dihexylpyrrolidinium, 1-hexyl-1-heptylpyrrolidinium, 1-hexyl-1-octylpyrrolidinium, 1-hexyl-1-nonylpyrrolidinium, 1-hexyl-1-decylpyrrolidinium, 1,1-dihexylpyrrolidinium, 1-hexyl-1-heptylpyrrolidinium, 1-hexyl-1-octylpyrrolidinium, 1-hexyl-1-nonylpyrrolidinium, 1-hexyl-1-decylpyrrolidinium, 1,1-diheptylpyrrolidinium, 1-heptyl-1-octylpyrrolidinium, 1-heptyl-1-nonylpyrrolidinium, 1-heptyl-1-decylpyrrolidinium, 1,1-dioctylpyrrolidinium, 1-octyl-1-nonylpyrrolidinium, 1-octyl-1-decylpyrrolidinium, 1-1-dinonylpyrrolidinium, 1-nonyl-1-decylpyrrolidinium or 1,1-didecylpyrrolidinium, 1-hydroxymethyl-1-methylpyrrolidinium, 1-hydroxymethyl-1-ethylpyrrolidinium, 1-hydroxymethyl-1-propylpyrrolidinium, 1-hydroxymethyl-1-butylpyrrolidinium, 1-(2-hydroxyethyl)-1-methylpyrrolidinium, 1-(2-hydroxyethyl)-1-ethylpyrrolidinium, 1-(2-hydroxyethyl)-1-propylpyrrolidinium, 1-(2-hydroxyethyl)-1-butylpyrrolidinium, 1-(3-hydroxypropyl)-1-methylpyrrolidinium, 1-(3-hydroxypropyl)-1-ethylpyrrolidinium, 1-(3-hydroxypropyl)-1-propylpyrrolidinium, 1-(3-hydroxypropyl)-1-butylpyrrolidinium, 1-(4-hydroxybutyl)-1-methylpyrrolidinium, 1-(4-hydroxybutyl)-1-ethylpyrrolidinium, 1-(4-hydroxybutyl)-1-propylpyrrolidinium or 1-(4-hydroxybutyl)-1-butylpyrrolidinium, 1-(1-hydroxymethyl)-3-methylimidazolium, 1-(1-hydroxymethyl)-3-ethylimidazolium, 1-(1-hydroxymethyl)-3-propylimidazolium, 1-(1-hydroxymethyl)-3-butylimidazolium, 1-(2-hydroxyethyl)-3-methylimidazolium, 1-(2-hydroxyethyl)-3-ethylimidazolium, 1-(2-hydroxyethyl)-3-propylimidazolium, 1-(2-hydroxyethyl)-3-butylimidazolium, 1-(3-hydroxypropyl)-3-methylimidazolium, 1-(3-hydroxypropyl)-3-ethylimidazolium, 1-(3-hydroxypropyl)-3-propylimidazolium, 1-(3-hydroxypropyl)-3-butylimidazolium, 1-(4-hydroxybutyl)-3-methylimidazolium, 1-(4-hydroxybutyl)-3-ethylimidazolium, 1-(4-hydroxybutyl)-3-propylimidazolium, 1-(4-hydroxybutyl)-3-butylimidazolium, 1,3-bis(1-hydroxymethyl)imidazolium, 1,3-bis(2-hydroxyethyl)imidazolium, 1,3-bis(3-hydroxypropyl)imidazolium, 1,3-bis(4-hydroxybutyl)imidazolium, 1-(2-hydroxyethyl)-3-(1-hydroxymethyl)imidazolium, 1-(2-hydroxyethyl)-3-(3-hydroxypropyl)imidazolium, 1-(2-hydroxyethyl)-3-(4-hydroxybutyl)imidazolium, 1-(3-hydroxypropyl)-3-(1-hydroxymethyl)imidazolium, 1-(3-hydroxypropyl)-3-(2-hydroxyethyl)imidazolium, 1-(3-hydroxypropyl)-3-(4-hydroxybutyl)imidazolium, 1-(4-hydroxybutyl)-3-(1-hydroxymethyl)imidazolium, 1-(4-hydroxybutyl)-3-(2-hydroxyethyl)imidazolium or 1-(4-hydroxybutyl)-3-(3-hydroxypropyl)imidazolium.
 7. Process according to claim 1, characterized in that selenium ions are in dissolved form in the ionic liquid.
 8. Process according to claim 7, characterized in that the selenium ions are produced by dissolution of a selenium salt.
 9. Process according to claim 1, characterized in that the substrate is a non-metal, semi-metal, metal, metal alloy or conductive and/or metallized ceramics or conductive and/or metallized plastic.
 10. Process according to claim 1, characterized in that the process is carried out at temperatures between 10° C. and 230° C.
 11. Substrates coated by a process according to claim
 1. 12. Use of ionic liquids for the deposition of grey selenium on a substrate. 